Transition flag from null to undefined (#831)

## Description:

Replace `null` with `undefined` for player flags.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

---------

Co-authored-by: Scott Anderson <662325+scottanderson@users.noreply.github.com>

src/core/Schemas.ts

@@ -430,7 +430,7 @@ export const ClientJoinMessageSchema = z.object({
   gameID: ID,
   lastTurn: z.number(), // The last turn the client saw.
   username: SafeString,
-  flag: SafeString.nullable(),
+  flag: SafeString.optional(),
 });
 
 export const ClientMessageSchema = z.union([